$0 Dr . Brewster’s experiments on the 
the faculty both of polarising and depolarising light, and the 
constant relation in the position of the axes which regulated 
these apparently opposite actions, induced me to think that 
the two classes of phenomena had the same origin. This 
opinion was afterwards strengthened by an experiment with 
a bundle of glass plates, in which light was depolarised by 
polarising it in a new plane ; but in applying the principle to 
other phenomena, I was baffled in every attempt to generalise 
them. By extending, however, and varying the experiments ; 
by examining the optical properties of every substance which 
I could command, and by comparing their structure with the 
phenomena which they exhibited, I have been led to the ge- 
neral principle to which they all belong, and to a series of 
results which, from their very nature, could not easily have 
been established by direct experiment. These conclusions, 
independently of their optical consequences, are peculiarly in- 
teresting to the chemist and the natural philosopher, by dis- 
closing the structure of organised substances, and exhibiting 
new relations among the bodies of the animal, the vegetable, 
and the mineral kingdoms. 
In proceeding to illustrate this subject, I shall first give 
some account of the experiments on which the theory is 
founded, and then explain the theory itself, and the conclu- 
sions which it seems to involve. 
I. Experiments on the depolarisation of light. 
I have already explained, in a former paper,* the general 
phenomena of depolarisation, and have shewn that almost all 
regularly crystallized bodies, such as plates of mica, of calca- 
* Philosophical Transactions, 1814, Part I, p. 199. 
